The present invention relates to a reflective liquid crystal display apparatus with a reflection electrode.
Previous reflective liquid crystal display apparatuses comprise a liquid crystal cell operating on TN or STN mode and sandwiched between a pair of substrates on which a transparent electrode has been formed, a pair of polarizers disposed in a manner such that they sandwich the liquid crystal cell, and a reflector disposed on the outer side of a polarizer on the lower substrate. However, in this configuration, as light passes the polarizers four times, the display is dark. Transmissivity of a polarizer is at most 45%. The transmissivity of polarized light parallel to the absorption axis of the polarizer is roughly 0%, while the transmissivity of polarized light perpendicular to the absorption axis is roughly 90%. Accordingly, the transmissivity of this configuration is calculated to be (0.9)4xc3x9750%=32.8% suggesting that reflectivity saturates at about 33% even for a black and while display panel. Especially, as a display of a color liquid crystal display panel in which color filter is formed on one of the substrates becomes darker than black and white display because of absorption by the color filter, it becomes difficult to secure brightness required of a reflective display. It also suffers inevitable parallax due to the thickness of substrates that exist between the reflector and the liquid crystal.
Accordingly, with a view to making the display brighter, some proposals have been made on configurations in which only one polarizer on the upper side of a liquid crystal cell is used and the liquid crystal cell is sandwiched between the polarizer and a reflector (for example, Japanese Laid-Open Patent Application No. Hei 7-146469 and Japanese Laid-Open Patent Application No. Hei. 7-84252). In these configurations, as light passes the polarizer only twice, the reflectivity of a black and white display panel becomes (0.9)2xc3x9750%=40.5% thus providing about 23.5% improvement in the reflectivity over configurations using two polarizers.
Also, with a view to making the display brighter, a reflective liquid crystal display panel of PCGH mode (phase-change type guest-host mode) has been proposed (ref. H. Seki: 1996 SID, p. 614, SID 96 Digest) as a configuration which does not use any polarizer at all. The reflectivity of this black and white display panel configuration is about 66% suggesting a bright display.
In the above described configurations either using a single polarizer or not using any polarizer at all, as a reflector can be disposed inside the liquid crystal cell by forming on the lower substrate a metal reflection electrode which works both as a reflector and an electrode, the issue of parallax can also be solved. It is practical that the-metal reflection electrode contains a film having Al that has a high reflectivity as the main ingredient (as in Japanese Patent Application No. Hei 9-208902, for example).
Also, in previous liquid crystal display apparatuses, an insulating film for electrode protection with a high-hardness value is formed between at least one of the electrodes and an alignment film. This insulating film for electrode protection prevents short-circuit between the upper and the lower electrodes caused by breakage of the alignment film due to foreign objects mixed in the liquid crystal cell or due to spacers. For advantages in productivity and cost, formation of the insulating film for electrode protection is performed by printing method in which a printed coat is heat treated by heating at 350xc2x0 C. or lower. Heat treatment at above 350xc2x0 C. will result in a higher resistance of ITO (indium tin oxide) that forms a transparent electrode. As the material for the insulating film for electrode protection, a composition containing silica oligomer, zirconia oligomer, or titania oligomer, for example, as the main ingredients is used. The composition is copolymerized by combined UV irradiation and heat treatment at about 300xc2x0 C. to obtain an inorganic insulating film with a high value of hardness. Also, in the formation of an insulating film for electrode protection onto a metal reflection electrode containing at least Al as the main ingredient, internal stress is produced in the metal reflection electrode due to heat-treatment, thus suffering a decrease in the bonding stress between the substrate and the metal reflection electrode. Accordingly, a proposal has been made to control the heat treatment temperature (for example, Japanese Laid-Open Patent Application No. Hei 10-216265).
A previous liquid crystal cell in which an insulating film for electrode protection is formed like this suffers defects in display caused by a slight change in the tilt angle due to scratches on the insulating film for electrode protection resulting from rubbing of the alignment film. This is because hysteresis in the electrical characteristic of the liquid crystal cell is produced as the voltage applied to the liquid crystal changes as charges are stored in the insulating film for electrode protection thus causing enlargement of a slight change in tilt of the alignment film into the form of electrical characteristic of the liquid crystal cell. Especially in a reflective liquid crystal display apparatus comprising an upper substrate on which a transparent electrode has been formed and a lower substrate on which an Al-containing metal reflection electrode has been formed, the standard electrode potential produced on the transparent electrode and the standard electrode potential produced on the metal reflection electrode are greatly different. As a result, charges stored in the insulating film for electrode protection increase thereby causing conspicuous defects in display.
The present invention addresses the above described problems of previous reflective liquid crystal display apparatuses and aims at providing a reflective liquid crystal display apparatus with a uniform display by controlling the defects in display due to storage of charges in the insulating film for electrode protection.
With a view to solving the above-described problems, the reflective liquid crystal display apparatus in accordance with the present invention comprises an upper substrate, a transparent electrode formed into an electrode pattern on one side of the upper substrate, an alignment film on the side of the upper substrate formed on the surface of the transparent electrode, a lower substrate disposed opposite to the upper substrate, a metal reflection electrode formed into an electrode pattern on the lower substrate and containing a film with at least Al as the main ingredient, an insulating film for electrode protection formed on the surface of the metal reflection electrode, an alignment film on the side of the lower substrate formed on the surface of the insulating film for electrode protection, and liquid crystals filled in the space between the upper and the lower substrates, and the volume resistivity of the insulating film for electrode protection is adjusted to 1xc3x97109 xcexa9xc2x7cm or smaller. With this configuration, the charges stored in the insulating film for electrode protection can be reduced and the defects in display due to the insulating film for electrode protection can be controlled.
Also, in another embodiment of the present invention, the volume resistivity of the insulating film for electrode protection is adjusted to 1xc3x97105 xcexa9xc2x7cm or greater. With this, a risk of short-circuit between the upper and the lower electrodes under a compressive load can be controlled thereby providing a reflective liquid crystal display apparatus which can realize a uniform display.
Also, in still another embodiment of the present invention, the metal reflection electrode is a dual-layer formed by laminating a Ti layer and an Al-alloy layer. By employing this configuration, a reflective liquid crystal display apparatus with improved electromigration resistance and corrosion resistance can be provided.
Also, in still another embodiment of the present invention, the upper and the lower substrates are made of soda lime glass, and an SiO2 film is formed between the electrode pattern and the substrates. Employment of this configuration allows a use of inexpensive glass substrates.
Also, in still another embodiment of the present invention, an inorganic insulating film is used as the insulating film for electrode protection. By employing this configuration, the risk of short-circuit between the upper and the lower electrodes can be controlled thereby providing a reflective liquid crystal display apparatus which can realize a uniform display.